Many digital radio communication systems, such as cellular, cordless and data transmission systems, use FSK, GFSK (Gaussian Frequency Shift Key), or GMSK (Gaussian Mean Shift Key) modulation techniques. These types of modulation techniques are in fact simply frequency modulation (FM) with the radio frequency (RF) signal envelope constant.
Since there is no amplitude modulation (AM) involved in these types of modulation techniques, the voltage controlled oscillator (VCO) frequency of the transmitters can be directly modulated by the baseband signal, as is typical in regular analog FM transmitters, such as in analog communications systems. Significant cost reductions can be obtained by directly modulating the VCO frequency. Such arrangements are particularly desirable in digital applications where low cost is a strategic factor. For example, the overall cost of a digital solution such as DECT must be very low to be competitive with well known analog systems such as CT0.
In order to avoid any inter symbol interferences which could corrupt the eyes pattern and degrade the bit error rate of the transmission in the digital system, the amplitude transfer function and the group delay on the modulation path has to be kept constant across the entire spectrum of the baseband signal. This requires the transfer function of the phase lock loop (PLL) to be high enough to pass the entire modulation spectrum. Furthermore, in order to meet the applicable radio specifications, the spectral purity of the RF signal source to be transmitted should be maintained as near to the carrier signal as possible for phase noise and modulation accuracy, and should be maintained as far from the carrier signal as possible to reduce harmonics, the noise floor, and discrete spurious signals. As a consequence, the transfer function of the PLL must be low enough to filter the noise.
A technique called Dual Port modulation is known to provide the capability to generate a low port modulation signal as well as a high port modulation. The low port modulation signal is typically used to drive a noise shaping circuit that controls the divider of the PLL, while at the same time the high port frequency modulation signal is used as a input to a high port path which utilizes a digital to analog converter (DAC) to directly drive the input voltage of a voltage controlled oscillator (VCO). The effect of the dual port modulation is to provide a low pass transfer function for the generated noise of the reference clock and the noise shaping circuitry, and to provide an all pass function to the input frequency deviation.
However due to practical manufacturing tolerances in components such as capacitors and inductors in these circuits, the modulation sensitivity response of the VCO across a range of transmission frequencies can be highly non-linear. The response is also affected by the method of coupling the high port signal to the VCO. The high port signal is typically coupled to the VCO at the loop filter such that it modulates the steering voltage to the VCO, or alternatively is used to modulate the capacitance of an external varactor which is coupled directly into the tank circuit of the VCO. The modulation sensitivity response for a simulated dual port frequency modulator having a high port signal coupled at the loop filter is shown in FIG. 1a. It can be seen that the modulation sensitivity of the VCO varies over a range of output frequencies. This is unacceptable for transmission devices and is typically addressed by “tuning” the circuit using variable gain or attenuation of the high port signal. By adjusting the gain of the high port signal, the modulation sensitivity can be flattened to an acceptable level prior to dispatch from the manufacturer.
The modulation sensitivity response of a simulated dual port frequency modulator having an external varactor modulated by the high port signal and coupled directly into the tank circuit of the VCO is shown in FIG. 1b. It can be seen that this also varies over a range of transmission frequencies and so this device arrangement also requires tuning using variable gain of the high port signal following manufacture.
The requirement for tuning adds to the time required to manufacture the frequency modulator units and hence their per unit cost. Also, as the modulation sensitivity can vary considerably over the transmission frequencies of interest, it may be necessary to significantly adjust the gain of the high port signal, requiring a large and expensive variable gain component in the modulator for this purpose.